Rare earth (e.g., elements having atomic numbers 57–71) doped optical fibers are known in the art to be useful in fiber amplifiers and lasers. In particular, Yb-doped fibers offer high output powers and excellent conversion efficiencies over a broad range of wavelengths (˜975 to ˜1200 nm). See, for example, R. Paschotta, J. Nilsson, A. C. Tropper and D. C. Hanna, “Ytterbium doped fiber amplifiers”, IEEE Journal of Quantum Electronics, 33(7), 1049–1056, 1997. In addition, unlike erbium doped amplifiers, complications such as excited state absorption and concentration quenching are avoided in Yb-doped fiber lasers and amplifiers. As a result, a high concentration of Yb ions can be incorporated while maintaining good conversion efficiencies. These attributes of Yb-doped fibers, along with the advent of double-clad fiber (DCF) technology, have resulted in substantial interest in high-power lasers and amplifiers for various applications. See, for example, L. Zenteno, “High-power double-clad fiber lasers”, Journal of Lightwave Technology, 11(9), 1435–1446, 1993. Yb-doped double-clad fibers are finding current and potential applications in military and aerospace, materials processing, printing and marking, spectroscopy, telecommunications, etc. See, for example, Paschotta et al. and Zenteno as referenced above, J. Noda, K. Okamoto and Y. Sasaki, “Polarization maintaining fibers and their applications”, Journal of Lightwave Technology, 4(8), 1071–1089, 1986, and J. P. Koplow, L. Goldberg, R. P. Moeller and D. A. V. Kliner, “Polarization-maintaining, double-clad fiber amplifier employing externally applied stress-induced birefringence”, Obtics Letters, 25(6), 387–389, 2000.
For many high-power laser and amplifier applications, operation under stable linear polarization is desirable. See Noda et al. and Koplow et al. as above. High-power amplifier (or laser) architectures are based on coherently combining the output of several DC fiber amplifiers. With the growing need for output powers of greater than 100 kW continuous wave (CW) for military and aerospace application and several kW outputs for industrial applications, there has been an increasing demand for polarization-maintaining double clad fibers (PM-DCF). Different approaches are known for obtaining PM operation using non-PM fibers. See, for example, Koplow et al. as above and I. N. Duling III and R. D. Esman, “Single-polarisation fibre amplifier”, Electronics Letters, 28(12), 1126–1128, 1992. However, these approaches have their limitations and the preferred technology is to use a PM-DCF. While passive polarization maintaining fibers have been commercially available for several years, active PM fibers have not been available until recently. See, for example, K. Tajima, “Er3+-doped single-polarisation optical fibres,” Electronics Letters, 26(18), 1498–1499, 1990 and D. A. V. Kliner, J. P. Koplow, L. Goldberg, A. L. G. Carter and J. A. Digweed, “Polarization-maintaining amplifier employing double-clad bow-tie fiber”, Obtics Letters, 26(4), 184–186, 2001. Kliner et al. were the first to report a polarization maintaining, Yb-doped, double-clad fiber amplifier employing a bow-tie fiber. Although a bow-tie type PM-DCF is acceptable for proof of concept and research and development, it has substantial limitations in terms of preform manufacturability, uniformity and scalability.
Single mode, Yb-doped, double-clad fibers lend themselves well to applications requiring compact lasers with diffraction-limited output. However, the scalability of output powers can be limited by amplified spontaneous emission and nonlinear processes such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS). These limitations can be overcome by using low numerical aperture (NA) single mode fibers with large mode areas (LMA). The low NA of the core limits the capture of the spontaneous emission by the core while the large mode area increases the threshold for SRS and SBS. In a second approach, multimode (MM) rare earth doped fibers can be used and the higher order modes suppressed by deploying the fiber in a specific coiled configuration (J. P. Koplow, D. A. V. Kliner and L. Goldberg, “Single-mode operation of a coiled multimode fiber amplifier”, Obtics Letters, 25(7), 442–444, 2000), optimizing launch conditions of the seed beam (M. E. Fermann, “Single-mode excitation of multimode fibers with ultra-short pulses,” Obtics Letters, 23(1), 52–54, 1998 and O. G. Okhotnikov and J. M. Sousa, “Flared single-transverse-mode fibre amplifier”, Electronics Letters, 35(12), 1011–1013, 1999), designing fibers with specific refractive index and dopant profiles (H. L. Offerhaus, N. G. Broderick, D. J. Richardson, R. Sammut, J. Caplen and L. Dong, “High-energy single-transverse-mode Q-switched fiber laser based on a multimode large-mode-area erbium-doped fiber”, Obtics Letters, 23(21), 1683–1685, 1998), and using specific cavity configurations (U. Greibner and H. Schonnagel, “Laser operation with nearly diffraction-limited output from a Yb-YAG multimode channel waveguide”, Obtics Letters, 24(11), 750–752, 1999). The use of a MM fiber in single mode operation provides similar advantages as the LMA fibers.